Traditional flat-field physical vapor deposition (PVD) of materials onto substrates containing high aspect ratio features, for example greater than 1:1, exhibits poor bottom and side-wall coverage, as well as void formation, in these features due to the wide angular distribution of the atoms being deposited from the sputter target onto the substrate, for example up to 90 degrees from the surface normal. This wide angular distribution occurs because the requirements of high deposition utilization, defined as the percentage of sputtered atoms deposited on the substrate, and good flat-field uniformity have traditionally necessitated a very short, i.e., less than one-third of the substrate diameter, target to substrate distance, also known as the throw distance. This short throw distance means that the solid angle subtended by the sputter target, at a given point on the substrate, can approach 2.pi. steradians. In other words, a point on the substrate "perceives" a target of infinite two-dimensional extent. While this is acceptable for depositing a blanket of material onto a flat field, it is highly undesirable for coating patterned substrates having high aspect ratio features due to shadowing of the incoming flux of atoms by the feature.
One method of narrowing the angular distribution of arriving atoms on the patterned substrate is to increase the throw distance. A conventional "long throw" PVD arrangement consists of a single planar cathode (sputter target) of larger diameter than the substrate having a throw distance greater than one substrate diameter. This arrangement is effective in decreasing the solid angle subtended by the target at every point on the substrate. However, for targets comparable in diameter to the substrate, achieving acceptable flat-field uniformity, for example less than 3% non-uniformity at one standard deviation, is only possible at an extremely large throw distance, for example.gtoreq.2 substrate diameters. Unfortunately, the deposition utilization is only several percent at these large throw distances, which is not practical for mass production applications. In addition, collisions of sputtered atoms with the ambient gas atoms/molecules between the target and substrate results in a broadening of the angular distribution of the sputtered atoms, which tends to defeat the purpose of a long throw distance. For these reasons, single planar target long throw techniques are limited to a range of throw distance that results in either unacceptably low flat-field uniformity or unacceptably low deposition utilization, or both.
Another drawback of long throw techniques is side-wall asymmetry, also known as inboard-outboard asymmetry. This means that one side of a feature is more heavily coated than the other side. This effect is a result of the fact that an off-center point on the substrate is bombarded by more atoms incident from the inboard side of the feature than the outboard side, assuming the center axes of the target and substrate are collinear. This asymmetry is usually most pronounced at the edge of the substrate. Increasing the target diameter does improve the asymmetry, but at the expense of broadening the angular distribution of the arriving atoms. This broadened angular distribution occurs because increasing the target diameter is geometrically equivalent to decreasing the throw distance.
Thus, the conventional "long throw" PVD technique utilizing a single planar cathode can effectively coat high aspect ratio features, but unfortunately with low flat-field uniformity and deposition utilization. To make the concept of "long throw" PVD practical, another technique is needed to provide a directional sputtered flux of atoms while still providing acceptable flat-field uniformity and deposition utilization.
In addition to the single planar cathode arrangement, other arrangements have been proposed for altering angular distribution, for example, U.S. Pat. No. 5,919,345. In this reference, it is proposed to provide a single non-planar convex or concave conical target. However, none of the proposed arrangements have proven satisfactory in achieving high flat-field uniformity and high deposition utilization.
There is thus a need for a PVD arrangement that provides an angular distribution of sputtered atoms that deposit with acceptable flat-field uniformity and deposition utilization.